Gas-filled diode discharge tube



Oct. 2, 1956 J. F. ZALESKI 2,765,445

GAS-FILLED DIODE DISCHARGE TUBE Filed Nov. 15, 1952 o I 1 f 2 h lq/maeas 52 l I l Z (deem I NVEN TOR.

JOHN F. Z191, ESK/ 5 W/ZuL GAS-FILLED DIODE DISCHARGE TUBE .lohn F.Zaleski, ihornwood, N. Y., assignor to General Precision LaboratoryIncorporated, a corporation of New York Application November 15, 1952,Serial No. 320,697

11 Claims. (Cl. 333-13) This invention relates to discharge tubes of thegasfilled cold-cathode type having two electrodes, and more specificallyto such tubes designed for use in microwave circuits such as modulators,mixers, attenuators or detectors.

The discharge tube of this invention in general consists of a disc orcone anode and a rod cathode coaxially disposed relative to the anodeand projecting through a central hole therein. The electrodes areenclosed in a glass envelope filled with gas at a suitable pressure, andprovision is made to minimize microwave energy leakage through the diodewhether ignited or unignited. Provisions for assembling the diode to amicrowave guide component are equally as important as the tubeconstructional details, since the tubes are designed for performance inor associated with microwave fields existing inside microwave guides.These assembly provisions include resonant and non-resonant means forassembling the tube in the narrow side, broad side or the end ofrectangular hollow guide, or in corresponding positions in round hollowor coaxial guide.

The principle underlying this invention utilizes the properties of theblue negative glow that occurs at the cathode of a glow discharge tube.This glow can be made to vary in exent or density by varying the tubecurrent, and these variations of the glow can be made to presentvariations in impedance to a microwave field impinging upon the glowplasma. Since the tube current can be controlled in the external tubepower supply circuit, such a discharge tube can be used to control ordetect microwave energy in a number of ways.

One purpose then of this invention is to provide convenient meansincluding a gas-filled diode discharge tube for efliciently modulatingmicrowave guided energy.

More specifically, a purpose of this invention is to provide agas-filled cold-cathode diode discharge tube for modulating microwaveenergy by changes in the impedance of its negative glow plasma.

Another purpose of this invention is to provide a gasfilled diodedetector for the demodulation of modulated microwave guided energy, suchdetector not being subject to burnout or other injury by the applicationthereto of energy of considerable magnitude, While affording highsensitivity.

Still another purpose of this invention is to provide a gas-filled diodemixer for microwave circuits.

Still another purpose of this invention is to provide an electricallycontrolled attenuator for use in microwave circuits.

A further understanding of this invention may be secured from thedetailed description and associated drawings, in which:

Figure 1 is a sectional view of the discharge tube of the inventionsecured in the narrow side of a rectangular microwave guide.

Figure 2 is a sectional view of the discharge tube secured by a holderto the broad side of a rectangular microwave guide.

States Patent ice Figure 3 depicts a microwave circuit illustrating useof the discharge tube of the invention.

Figures 4 and 5 are graphs depicting characteristics of glow discharges.

Referring now to Fig. 1, a diode discharge tube is provided with ananode 11 in the form of a flat conductive disc or ring made of a metalwhich can be welded to glass and which has a low surface resistivity formicrowave energy. This anode is provided with a central aperture 12. Acathode 13 consisting of a straight rod of the same metal as the anodeis positioned coaxially with the anode 11, extending through theaperture 12 therein. The rod cathode 13 is preferably thin and round inorder to present minimum disturbance of the microwave field surroundingit and to increase the direct-current voltage gradient at its surface,promoting voltage breakdown and initiation of the glow discharge. Therod cathode 13 may be coated with electron-emitting compounds, and isprovided with an axial bore extending from its end 14 to a point nearits other end, a transverse port 16 being provided at this point forcommunication between the axial bore andthe space exterior to the rod13. The axial bore and the port 16 serve in exhausting the tube, andafter exhaustion the tip 14 is sealed off. The rod cathode 13 and discanode are bound together mechanically, while insulated electrically, bya gas-tight glass seal 17. The left end of the cathode, as seen in Fig.1, is enclosed in a gas-tight glass envelope 18, sealed to the tip ofthe cathode at 19 and sealed to the anode 11 in a ring seam indicated at21. The left end of the cathode preferably projects through the seal 19so that the metal end of the cathode projects beyond the glass envelope,as this construction makes the uniformity of capacitance of theprojecting tip easier to control in manufacture. The envelope 18 isfilled through tubulation 14 with an appropriate filling gas at suchpressure as to permit use of the diode in the glow discharge region, andat a part of that region at which the negative glow plasma of the glowdischarge otters low resistance to microwave field energy. For example amixture of neon and less than 1% of argon at a pressure of between 1 and10 mm. of mercury is suitable.

The diode as so far described is suitable for general application tomicrowave energy control and detection in association with a variety ofmicrowave guides. In Fig. 1 the diode is shown applied to the narrowside of a microwave hollow rectangular guide 22 for transmittingmicrowave energy of a selected wavelength or band of wavelengths, withthe rod cathode 13 passing nearly or not quite across the guide, midwayof the broad sides and perpendicular to both the axial line of the guideand the direction of the electric field component of the microwaveenergy carried by the guide. The length of the rod 13 should not,however, equal one-quarter of the free space wavelength or of any of theband of wavelengths carried by the guide 22, or any odd multiplethereof, because such length of rod even when perpendicular to thedirection of the microwave electric field exhibits resonance effects andwill both absorb and reflect microwave power to some extent.

In order to permit insertion of the discharge tube into the guide therectangular guide 22 is provided in one of its narrow sides 23 with around opening having a diameter equal to the smaller cross-sectionaldimension b of the guide. A circular disc or flange of metal 24 having acentral bore of diameter b is secured to the outside surface of thenarrow side 23 so that its central bore surrounds the opening in thenarrow side of the guide. The outer edge of the flange 24 is threaded at26. A cap the threads 26 of the flange and the anode disc 11 is seatedin a recess 28 so that the anode disc 11 is clamped between bottom ofthe recess and the flange 24.

In order to prevent escape of the microwave energy from the wave guide22 through the aperture 12, a metal disc 29'is provided snugly securedto the rod 13 and extending well beyond the edges of the aperture 12.This disc 29 is insulated from the anode 11 for direct current by aninsulating disc 31 which may be of mica. The disc 29 thus serves as anelectrostatic shield at the aperture 12 while the combination of thedisc 29, insulator 31 and anode plate 11 forms a condenser thateffectively bypasses the aperture 12 at microwave frequencies.

The diode is suitable for use in the rectangular guide as shown as amodulator, mixer, attenuator and demodulator, employing suitablecircuits such as are well known in the art.

The diode may be applied to a rectangular guide and perform its intendedfunction even though the diode does not project appreciably into theguide or even when the diode does not enter the guide at all. In suchinstances a holder is attached to the external surface of a broad guideface, with an aperture connecting the interior of the holder with theinterior of the guide. An iris opening may be employed at the aperturefor further control, and the length of the holder may be made so thatthe holder together with the contained discharge tube may benon-resonant, resonant, or anti-resonant at the microwave frequencyemployed in the guide.

A preferred form of holder is shown in Fig. 2 containing a dischargetube similar to that of Fig. 1. The discharge tube contains anode 11,rod cathode 13, and envelope 18, with glass welds at 17, 19 and 21.Protection against escape of microwave energy is provided by aprefabricated button-type condenser 32 pressed on to the cathode rod 13so that its central eyelet terminal 33 is in good electrical contacttherewith as well as in contact with one condenser plate 30. Theperipheral rim terminal 34 is in electrical contact with the othercondenser plate or plates 35 and with the anode disc 11 of the diode.This condenser may be of either the ceramic or silver-mica variety, forexample, and its capacitance is not critical. As an example of itselectrical size, a button condenser having a capacitance of 100 if. hasbeen found to prevent the escape of any measurable quantity of microwaveenergy at a frequency of 10,000 me. p. s.

The discharge tube is fastened in a holder including a cylinder 36having an internal diameter greater than that of the envelope 18 but notgreater than the larger internal cross sectional dimension of the guide37, "upon which guide face the cylinder is secured by screws 38 passingthrough a flange 39 threaded into the guide. The area of the guidesbroad side encompassed by the cylinder is partly or entirely cut away toprovide access for microwave field energy between the interior of theguide and the interior of the cylinder. The anode ring 11 rests upon thupper end 41 of the cylinder, where it is secured by a screw thimble 42in threaded engagement at 43 with the upper end of the cylinder 36. Thecylinder 36 together with the cathode rod 13 thus forms a reentrant orcoaxial cavity closed at one end by the disc anode 11 and open or partlyopen at the other end to the interior of the guide. It is preferablethat the effective length L of this cavity be any integral multiple ofone-half of the microwave length of the energy existing in the coaxialcavity. The open end of the cavity then presents no obstacle to passageof microwave energy through the guide.

This combination of discharge tube and holder is suitable for use as amodulator, mixer, attenuator and detector.

Forms of by-pass condenser other than those illustrated in Figs. 1 and 2can be used to prevent the escape of microwave energy from the guide,and other structures well known in the art can be employed. In all casesit is required that the escape of microwave energy be prevented bothwhen the tube is unfired or unenergized by the direct current ormodulating source, and when it is fired or is in the glow dischargecondition. The requirements also indicate a compact and easily madedesign which does not interfere with the operation of the glowdischarge.

It is obvious that the discharge tubes of this invention can be employedfor the mentioned purposes not only in hollow rectangular guide and incoaxial guide, but also in round guide carrying microwave energy of theproper mode and phase so that operation as described may occur.

7 Operation as a modulator or mixer of the type of discharge tube andholder shown in Fig. 2 in the broad side of a rectangular wave guide isillustrated in Fig. 3. A microwave generator 44 generates microwaveenergy of a selected wave length and applies it for transmission througha rectangular hollow wave guide 46 to a nonrellective load 47. Adischarge tube and holder such as shown in detail in Fig. 2 is indicatedat 48. A directcurrent source of potential sufficient to start a glowdischarge in tube 48 is indicated by the battery 49 This potentialsource is current amplitude modulated in any desired manner by amodulator 51 and the output is applied to tube 48, the rod cathode 52being connected to the negative terminal of the modulator.

Returning now to Fig. 2, when direct current potential is appliedbetween anode and cathode which is above the sparkover potential, withthe current flow controlled, a self-sustained negative glow appears atthe point where anode and cathode most nearly approach each other. Ifcurrent be restricted to a value normally in the neighborhood of 1 ma.,the glow discharge will be confined to a short length of the cathode rodnear the anode. As the current is increased, the negative glow creepsalong the cathode in proportion, until at some current value i, the glowreaches the end of the cathode terminating at the seal 19. If thecurrent be further increased, the current density per unit area of rodincreases, probably by both increase in diameter and increase in densityof the plasma constituting the negative glow. Finally, at some value ofcurrent, usually above 40 ma., the character of the discharge changesfrom a glow to an arc. This volt-ampere characteristic is illustrated inFig. 4, the region from i to i being the glow discharge region, and theregion above 1' being the arc discharge region.

When the discharge tube of this invention is operated in this arecharacteristic region, it has been found that the tube is an excellentnoise generator. The tube can therefore be employed in an attenuatorcircuit for generating and introducing noise spectrum modulatingfrequencies into a microwave circuit.

When the tube is operated in the glow discharge region the currentdensity per unit length of that part of the cathode that is covered withthe glow discharge is shown in Fig. 5. The left part of this curve ishorizontal and indicates constant glow density per unit area of rod asthe glow creeps out along the rod. The current value i marks theextension of the glow to the extreme end of the rod, this current valueterminating the horizontal part of the curve. Further input currentincrease then increases the current density and the curve bends upward.The horizontal part of this curve represents what is sometimes calledthe normal glow discharge, while the curved part represents theso-called abnormal glow discharge. The design of the instant dischargetube as used in Fig. 3 exploits the normal glow discharge.

Let it be supposed that in the use of the discharge tube and holder ofFig. 2 in the circuit of Fig. 3 such average modulating current isapplied to the tube as to cause a negative glow to sheathe the rodcathode 13 for one-half of its length. The resistance of the plasma issuch as to make the cavity absorb some microwave power. Therefore, asthe cavity, tuned to the frequency of the microwave energy passingthrough the guide, abstracts energy from the guide, the cavitydissipates some of the abstracted energy and returns the remainder tothe guide. This energy-absorbing property of the cavity is highlysensitive to the length of the luminous sheath surrounding the rodcathode. This sheath length is in turn controlled by and directlyproportional to the applied current and if, as here, the current ismodulated in any manner, the modulations will appear as modulations ofthe sheath length, in turn very efficiently amplitude-modulating themicrowave energy that is transmitted through the guide past the cavity.Thus by making use of the change in length of the negative glow sheathalong the rod cathode 13, the discharge tube in association with themicrowave structure of Fig. 2 constitutes an efficient modulator ofmicrowave energy.

It is obvious that any modulator of this type operating on the principleof energy absorption can serve as an attenuator. The loss of energy inguide 37, Fig. 2, is a function of the direct current applied to thedischarge tube, and therefore if in Fig. 3 a simple current control besubstituted for the modulator 51, the circuit becomes that of anattenuator.

The operation of the discharge tube is reversible in the sense that thestrength of the microwave electric field surrounding the plasma and incontact with it has an effect upon the apparent impedance of the plasmaand therefore upon the voltage drop across it at a selected currentflow. The discharge tube can therefore be used as a detector ordemodulator.

What is claimed is:

l. A gas-filled negative glow diode discharge tube for use with awaveguide comprising, an elongated cold cathode, a fiat anode connectedto said waveguide and containing an aperture, said cathode having anextension projecting through the aperture in said anode toward saidwaveguide, an insulating gas-tight envelope sealed to said anode andenclosing said cathode, gas at low pressure filling said envelope, andmicrowave means surrounding said cathode and positioned contignlous tosaid aperture for preventing the passage of microwave energy throughsaid aperture.

2. A gas-filled negative glow diode discharge tube for insertion in amicrowave field comprising, a rod-shaped cold cathode, a conductiveannular anode containing an aperture, said cathode being provided withan extension projecting through said aperture, an insulating gastightenvelope sealed to said anode and enclosing said cathode, a gas fillingfor said envelope, said gas filling having such composition and pressureas to permit the formation of a negative glow on said cathode upon flowof electric current between anode and cathode, the conductance of saidnegative glow for microwave field energy varying with said current, andmicrowave means surrounding said electrode and positioned contiguous tosaid aperture for preventing passage of microwave energy through saidaperture.

3. A gas-filled negative glow diode discharge tube comprising, anelongated conductive electrode, a conductive anode in the form of asolid of revolution provided with a central aperture coaxiallysurrounding said electrode, one end of said electrode extending axiallyaway from said anode to form a cold cathode, an insulating gastightenvelope completely surrounding contiguous portions of said anode andelectrode, ionizable gas filling in said envelope capable of forming anegative glow plasma around said cathode, means for immersing saidcathode in a microwave field having a selected frequency, and microwavemeans surrounding said electrode and positioned contiguous to saidaperture for preventing passage of microwave energy through saidaperture.

4. A gas-filled negative flow diode discharge tube and holdercomprising, a rod-shaped conductive electrode, a conductive annularanode surrounding but spaced from said electrode, one end of saidelectrode extending axially from said anode to form a cold cathodecapable of emitting electrons upon bombardment with positive ions, aninsulating gas-tight envelope completely surrounding contiguous portionsof said anode and electrode, a modulated direct-current source connectedbetween said anode and electrode, a gas filling within said envelopehaving such nature and pressure as to be ionized under electric tensionby electron collision to form a negative glow plasma surrounding saidcathode, the amount of said negative glow plasma varying with saidmodulation and the effective conductance of said plasma for microwavefield energy varying with said amount of plasma, microwave meanssurrounding said electrode and positioned contiguous to the junction ofsaid anode and electrode for preventing egress of microwave energythrough the region of nearest approach of said anode to said electrode,a microwave guide, and a conductive holder surrounding said cathode andenvelope and connected to said anode for attaching said discharge tubeto said microwave guide, said holder together with said discharge tubeforming a microwave reentrant cavity.

5. A discharge tube and holder in accordance with claim 4 in which thedimensions of said microwave resonant cavity are such that the cavity isresonant to microwave energy applied thereto from said microwave guide.

6. In a microwave component a gas-filled negative glow diode dischargetube for immersion in a microwave field comprising, a rod-shapedconductive electrode, a conductive annular anode coaxially surroundingbut spaced from said electrode, one end of said electrode extendingcoaxially in one direction from said anode to form a cold cathodecapable of emitting electrons upon bombardment with positive ions, theother end of said electrode extendin through said anode in the otherdirection to form a cathode terminal, an insulating gas-tight envelopeextending from said anode to the vicinity of the distal end of saidcathode, a glass seal connecting the nearest parts of said electrode andanode and electrically insulating them from each other, means forapplying direct current between said cathode terminal and said anode, agas filling for said envelope at such low pressure as to be ionized bysaid current flow to form a negative glow surrounding said cathode saidnegative glow having relatively low impedance at microwave frequenciesand having a short-circuiting effect upon a microwave field impingingthereon, while the potential of a microwave field impinging thereonaffects the direct-current potential drop across said anode and cathode,and microwave means surrounding said electrode and positioned contiguousto the junction of said anode and electrode for preventing the egress ofmicrowave field energy through the region of nearest approach of saidanode and electrode.

7. A discharge tube in accordance with claim 6 in which said microwavemeans for preventing egress of field energy includes a by-passcondenser.

8. A diode gas discharge tube comprising, a fiat disc shaped anodeprovided with a central aperture, a rod shaped cathode projectingthrough said aperture hermetically and insulatably sealed thereto, aglass envelope sealed at one end to one face of said anode extendingaxially of said cathode to enclose the major portion thereof, the distalend of said cathode projecting through the end wall of said envelope andbeing hermetically sealed thereto, means surrounding said cathode andpositioned contiguous to the aperture in said anode for preventingtransmission of microwave energy therethrough, and a gas filling forsaid envelope.

9. A diode gas discharge tube comprising, a fiat disc shaped anodeprovided with a central aperture, a rod shaped cathode projectingthrough said aperture hermetically and insulatably sealed thereto, aglass envelope sealed at one end to one face of said anode extendingaxially of said cathode to enclose the major portion of the end of saidcathode projecting from said one face of said anode, the distal end ofsaid cathode projecting through the end wall of said envelope and beinghermetically sealed thereto, a microwave bypass condenser postionedadjacent said anode central aperture electrically interconnecting saidanode and cathode, and a gas filling for said envelope.

10. A diode gas discharge tube comprising, a fiat disc shaped anodeprovided with a central aperture, a rod shaped cathode projectingthrough said aperture hermetically and insulatably sealed thereto, agenerally cylindrical glass envelope positioned concentrically withrespect to said cathode and sealed at one end to said anode in anannulus interme' iate the radial extent thereof, a conductive plateconnected to and surrounding said cathode positioned adjacent the faceof said anode opposed to that to which said envelope is sealed, a sheetof dielectric material interposed between said anode and conductiveplate, and a gas filling for said envelope,

ll. A diode gas discharge tube comprising, a flat disc shaped anodeprovided with a central aperture, a rod shaped cathode projectingthrough said aperture hermetically and electrically insulatably sealedthereto, a generally cylindrical glass envelope positionedconcentrically with respect to said cathode and sealed at one end tosaid anode in an annulus intermediate the radial extent thereof, amember postioned on said cathode adjacent the face of said anode opposedto that to which said envelope is sealed, said member comprising atleast one dielectric disc separating at least two conductive discs inmutual capacitive relation, said member having a conductive ring aroundits periphery and a metallic center eyelet, said eyelet contacting atleast one of said conductive discs and said cathode, and a gas fillingfor said envelope.

References Cited in the file of this patent UNITED STATES PATENTS2,644,139 Hunter June 30, 1953

